An extraordinary meteorological event; was one of its results a 1000-year flood?

A remarkable combination of a deep cutoff low aloft, an old front and a new one developing with a non-tropical surface low, a potent low-level jet, very strong ridging aloft and surface high pressure to the north, and extra moisture/fetch contributed by Hurricane Joaquin (whose steering was affected by a system that was associated with the remnant of Ida), resulted in an extraordinary convective band, extreme rainfall and disastrous flooding in the southeastern United States; wind damage from gusts and wet soil with loosened tree roots; and, from parts of the Northeast down across the Mid-Atlantic to the Southeast, coastal flooding and severe beach erosion from a persistent onshore wind gradient through many astronomical tide cycles.

There have been many other cases of repetitive convection in various places, Pineapple Express type atmospheric rivers into the West Coast, and persistent lake-effect snow squall bands, but I've been forecasting the weather since the 1970s and I can't remember having seen anything in the southeast U.S. or anywhere else quite like the nature of this long solid discrete relentless band of torrential thunderstorms training (from ESE to WNW no less) hour after hour after hour after hour after hour after hour after hour (the loop is 7 hours, and the band was going long before and long after that), along with the combination of synoptic ingredients that existed.

Certainly the event was exceedingly rare. But exactly how unusual? In particular, was this a "1000-year flood"?

Such a specific description can be problematic in a number ways.

● An X-year rainfall is not necessarily an X-year flood, as a number of factors affect how a given rainfall total translates into impacts.

● That type of term itself is misunderstood, as it's not meant to imply that such events occur every X years, it's that they occur with that frequency on average, and thus there's a particular probability in any given year of such an occurrence. But the way it appears is usually not a headline or soundbite of "A 0.1% Probability of A Flood Like This in Any Given Year," it's "1000-year Flood!" (And that's reported as if it's an absolute fact.) And in regard to precision and rounding off, do the statistical calculations equal precisely 1000 years? Or 950? 1098?

● What are the spatial characteristics of a particular "rainfall event"? An extreme amount that happens to fall at a particular location for which recurrence intervals are calculated? Or extreme amounts collectively, covering an area like this in the graphic below? And given that X-year floods/storms can occur in multiple places in a given year, that further complicates the issue. Furthermore, now that there is an easily-accessible calculator, to what extent is that affecting communication and perception?

● The frequency can be established for events that occur within a known period of record measurement. For example, let's say that 10" of rain has fallen in 24 hours, at a given location where such observations have been recorded since 75 years ago, six times during that period of record. We can say that since 1940, that amount of precipitation has occurred at an average frequency of eight times per century. For longer return intervals, there are statistical methods that are used to extrapolate. But what if during the period of record, no such event has occurred. How accurate is an extrapolation? And do we really know what the frequency has been over the course of centuries and millennia, without the data? No. How many times have 10-20+" across that size of an area fallen in South Carolina during the past 10,000 years? Were there storms in October 1011 or August 827 B.C. that brought such a deluge? Or could this be a 2000-year flood? And I'm not being flippant, I'm asking serious questions.

● And what if atmospheric water vapor and circulation patterns are changing, resulting in a recent trend in the frequency of extreme precipitation events such as that below? Is an X-year rainfall still an X-year rainfall? For that matter, how will this weekend's event change the average recurrence interval calculations in South Carolina?

Quoting 13. moneymike: the actual years that it would take for this flood to repeat is 7598 years according to NOAA own 1000 year flood calculator. I kept adding and subtracting years until I hit 100 percent and thats the number it kicked out.

You actually are not using that tool correctly. That is not a correct conclusion to reach based upon that information.

At best, you could say that for any one particular point location within the hardest hit area, rainfall of that magnitude had <0.1% chance of occurring in any given year. The term "average recurrence interval" doesn't say anything about how much time to expect before the next event, all it tells us is that over very long periods of time, the interval averages about that long. Because the interval can widely vary between small periods and long periods, it really isn't that useful, which is why using the annual chance is a much better way to describe extreme events.

the actual years that it would take for this flood to repeat is 7598 years according to NOAA own 1000 year flood calculator. I kept adding and subtracting years until I hit 100 percent and thats the number it kicked out.I have been a weather observer for 10 plus years. I was in the blizzard of oct31- nov1 1991 minesota snow storm the boston 1978 snow storm i and 70 below in antarctica and this rain was scary Sitting in your house watching water come in and deciding whether to stay or go. I stayed I recieved minor flood damage in the wierdest way . It was due to my neighbor planting a tree and the water back up went under the slab and into the house.

While this certainly was a catastrophic flood with lots of damage and tragic loss of life, USGS provisional data and preliminary analysis show NO indication that a 1000-year flood discharge occurred at any USGS streamgages.

Is this flood due to climate change?

USGS research has shown no linkage between flooding (either increases or decreases) and the increase in greenhouse gases. Essentially, from USGS long-term streamgage data for sites across the country with no regulation or other changes to the watershed that could influence the streamflow, the data shows no systematic increases in flooding through time.

A much bigger impact on flooding, though, is land use change. Without proper mitigation, urbanization of watersheds increases flooding. Moreover, encroachment into the floodplain by homes and businesses leads to greater economic losses and potential loss of life, with more encroachment leading to greater losses.

...oops wrong blog(was posting realtime NYC wx obs) , though i am reading it cause i like Mr Ostro style of informing...might as well post something related it a VID and slideshow of Joaquin and to JQs west the "firehose" effect onto the Carolinas VID (apology for the image bounce, the SAT imagery had superimposing errors.)::https://youtu.be/a6D9Q0-qOVA(600x320, has 28x28 top-bottom padding)

and the slideshow might not work and could not finish uploading the 68 more frames as i reached the popularity limit for a free account, SLIDESHOW LINK (not embeddable on WxU) http://www.cinco.ly/~AcEAR-cB_MNt

I am curious about the circumstances of the heavy rains in Charleston in June 1973. There was no tropical cyclone activity in the Atlantic during June 1973. As you can see from the monthly extremes at KCHS, there is a gradual progression upwards in monthly totals up to #2, with June 1973 having a monthly total almost 9" above the #2 month.

Quoting 7. Hydrargyrum: The problem with the way the rainfall and flooding in South Carolina have been reported is that the National Weather Service says the probability of such flooding is 0.1%, or one chance in 1000, but in the minds of the mainstream media it gets translated into a 1000-year flood, so that's what the public hears. There's no natural law that prevents more than one episode of heavy rainfall in a single year; the flooding we've seen could happen at random intervals averaging considerably less than 1000 years.

I watched an interesting video clip a few days ago where a TV meteorologist was using a wide-area, color-enhanced time lapse satellite image of Hurricane Joaquin, showing the low level jet emanating from the center of the storm. As the storm headed north, the direction of the jet changed, keeping it constantly pointed at the Carolinas, almost as if it had been intelligently controlled with the intention of concentrating as much rain in the region and causing as much damage as possible. I'd say the chances of such a weather formation persisting for so long might be less than one in 1000.

The other point missed is that 100, 1000, 50, etc year events are defined by the return period at points. Nearby areas may get one when your point doesn't. This is common with flash flood events from thunderstorms and I expect someone within 50 miles of me to get unlucky and get a 100 year dump about one year in two. I also seem to be unlucky and have had several 4"+ rain events in a day here in DC and a few 8"+ events (may 11/12 2008, also late June 2006) Most recently I got 4" in an hour June 10, 2014. Flooding in my area was massive! Some points about 50 miles to my southwest got a foot of rain in 12 hours in Sept 2011 from TS Lee remnants . Another storm produced 3.5" in an hour June 2009 about three miles to my north. I got less than an inch.

The problem with the way the rainfall and flooding in South Carolina have been reported is that the National Weather Service says the probability of such flooding is 0.1%, or one chance in 1000, but in the minds of the mainstream media it gets translated into a 1000-year flood, so that's what the public hears. There's no natural law that prevents more than one episode of heavy rainfall in a single year; the flooding we've seen could happen at random intervals averaging considerably less than 1000 years.

I watched an interesting video clip a few days ago where a TV meteorologist was using a wide-area, color-enhanced time lapse satellite image of Hurricane Joaquin, showing the low level jet emanating from the center of the storm. As the storm headed north, the direction of the jet changed, keeping it constantly pointed at the Carolinas, almost as if it had been intelligently controlled with the intention of concentrating as much rain in the region and causing as much damage as possible. I'd say the chances of such a weather formation persisting for so long might be less than one in 1000.

The thing that bothers me about the whole "thousand year flood" hype is that the argument is entirely statistical, and the statistics are based on a fairly small sample. I would be more interested/impressed if meteorologists actually had a thousand years (or, better yet, several thousand years) of data in the record.

Of course, I guess that's pretty much your point here. Ten thousand years of data would support definitive statements about, for example, global warming as well. Too bad we'll just have to wait.

Shirley the earth is only 6500 years old, so this event could be the worst since the beginning of the history of the earth. And yes, I am being flippant.More seriously, thank you Stu for the analysis. I have been forecasting only since the 1990s but the moment I saw the satpic and hurricane track I though it just looked climatically, even meteorologically wrong. But you can't deny the facts.The bigger question remains, what were the macro conditions that allowed it? How much influence has El Nino had? Et cetera.

Though the dynamics were certainly different, this does remind me of the extended fetch of wind off lake Erie last winter that piled up 100 inches of snow in a narrow band south of Buffalo. The right conditions in place and remaining so for a long period.

Great blog, Stu. You pose worthy questions, and I'm particularly interested in your final bullet. I fear that before we are able to adequately answer questions like these, we will already have caused irreversible and potentially catastrophic damage.